In a communication system, a transmitting apparatus is used to convert a useful signal into an output signal. For this purpose, a frequency signal is modulated based on information comprised in the useful signal. The output signal is transmitted in the form of a transmission signal via a communication channel, such as a data cable or a radio channel. In a receiver, the received transmission signal is demodulated to provide the restored information in a form which enables further signal processing.
In the communication channel, a multiplicity of interferers distorts the transmission signal. These effects may include thermal noise present in the transmitting apparatus. Other transmission signals in the communication signal or interference signal of electrical devices are likewise responsible for interfering with or degrading the transmission signal. Another source of signal degradation is attenuation of the transmission signal due to spatial separation of the transmitting apparatus and the receiver. For successful transmission of the information contained in the useful signal, the output signal has to be provided at a signal level sufficiently large for the received transmission signal to be processed by the receiver.
Modern communication systems, and in particular wireless communication systems, have a large dynamic range for the signal level. This is the case, inter alia, for amplitude-modulated transmission systems. One example is a communication system defined in the so-called Universal Mobile Telecommunication Standard (UMTS). Another example is the so-called High Speed Uplink Packet Access (HSUPA) Standard.
The RMS power (root mean square power) of the transmitting apparatus is used to indicate the average power level of the transmitting apparatus without interference or overdriving. In a UMTS or HSUPA system, said power is approximately 35-40 dB in relation to a minimum output power required. In addition, the different standards impose additional demands on the power level of the transmitter of up to 80 dB. Overall, the dynamic range of a transmitter using amplitude modulation may range between 115 dB to 120 dB.
In known transmitting apparatus, an amplifier unit adjusts the transmission signal at the required output power. E.g., in polar transmitter architecture, the output power may be set in an amplitude branch of the transmitter. If necessary, a further setting operation is carried out by controlling an operating point of an amplifier in the phase branch of the transmitter. For the case of so-called polar modulator architecture, the power may be adjusted by setting the output power in the amplitude branch of the transmitter. If necessary, provision is made for a further setting operation by means of pre-amplification, e.g. by a controlled amplifier or a variable gain amplifier (VGA). In all known architecture, the achievement of a large dynamic range is associated with particular expenditure. E.g., when a VGA is used, setting of the output power requires additional control of the temperature response of the VGA.